A spark plug for providing ignition in an internal combustion engine, such as a gasoline engine, has the following structure: an insulator is provided externally of a center electrode; a metallic shell is provided externally of the insulator; and a ground electrode which forms a spark discharge gap in cooperation with the center electrode is attached to the metallic shell. The metallic shell is generally formed from an iron-based material, such as carbon steel, and, in many cases, plating is performed on its surface for corrosion protection. A known technique associated with such a plating layer employs a 2-layer structure consisting of an Ni plating layer and a chromate layer (See Japanese Patent Application Laid-Open (kokai) No. 2002-184552). However, the inventors of the present invention have found that, even in employment of a plating layer having such a two- or more-layer structure, corrosion resistance is still a big problem for a portion of a spark plug which is deformed at the time of crimping. The following description first discusses an example structure of a spark plug and a crimping step, and then a portion of the spark plug which is deformed from crimping and involves a problem with respect to corrosion resistance.
FIG. 1 is a sectional view of essential members, showing an example structure of a spark plug. A spark plug 100 includes a tubular metallic shell 1; a tubular insulator 2 (ceramic insulator), which is fitted into the metallic shell 1 in such a manner that its forward end portion projects from the metallic shell 1; a center electrode 3, which is provided in the insulator 2 in such a state that its forward end portion projects from the insulator 2; and a ground electrode 4, whose one end is joined to the metallic shell 1 and whose other end faces the forward end of the center electrode 3. A spark discharge gap g is formed between the ground electrode 4 and the center electrode 3.
The insulator 2 is formed from, for example, a ceramic sintered body of alumina or aluminum nitride and has a through hole 6 formed therein in such a manner as to extend along the axial direction thereof, and adapted to allow the center electrode 3 to be fitted therein. A metal terminal 13 is fixedly inserted into the through hole 6 at a side toward one end of the through hole 6, whereas the center electrode 3 is fixedly inserted into the through hole 6 at a side toward the other end of the through hole 6. A resistor 15 is disposed, within the through hole 6, between the metal terminal 13 and the center electrode 3. Opposite end portions of the resistor 15 are electrically connected to the center electrode 3 and the metal terminal 13 via electrically conductive glass seal layers 16 and 17, respectively.
The metallic shell 1 is formed into a hollow, cylindrical shape from a metal, such as carbon steel, and forms a housing of the spark plug 100. The metallic shell 1 has a threaded portion 7 formed on its outer circumferential surface and adapted to mount the spark plug 100 to an unillustrated engine block. A hexagonal portion 1e is a tool engagement portion which allows a tool, such as a spanner or a wrench, to be engaged therewith in mounting the metallic shell 1 to the engine block, and has a hexagonal cross section. The tool engagement portion may have any cross-sectional shape (orthogonal-to-axis sectional shape) other than a hexagonal shape; for example, the tool engagement portion may have another polygonal cross section, such as an octagonal cross section. In a space between the outer surface of the insulator 2 and the inner surface of a rear (upper in the drawing) opening portion of the metallic shell 1, a ring packing 62 is disposed on the rear periphery of a flange-like projection 2e of the insulator 2, and a filler layer 61, such as talc, and a ring packing 60 are disposed, in this order, rearward of the ring packing 62. In assembling work, the insulator 2 is pressed forward (downward in the drawing) into the metallic shell 1, and, in this condition, the rear opening end of the metallic shell 1 is crimped inward toward the ring packing 60 (and, in turn, toward the projection 2e, which functions as a receiving portion for crimping), whereby a crimp portion 1d is formed, and thus the metallic shell 1 is fixed to the insulator 2.
A gasket 30 is fitted to a proximal end of the threaded portion 7 of the metallic shell 1. The gasket 30 is formed by bending a metal sheet of carbon steel or the like into the form of a ring. When the threaded portion 7 is screwed into a threaded hole of the cylinder head, the gasket 30 is compressed in the axial direction and deformed in a crushed manner between a flange-like gas seal portion 1f f of the metallic shell 1 and a peripheral-portion-around-opening of the threaded hole, thereby sealing the gap between the threaded hole and the threaded portion 7.
FIG. 2 is an explanatory view showing an example step of fixing the metallic shell 1 to the insulator 2 through crimping (FIG. 2 omits the illustration of the ground electrode 4). First, as shown in FIG. 2(b), the insulator 2 whose through hole 6 accommodates the center electrode 3, the electrically conductive glass seal layers 16 and 17, the resistor 15, and the metal terminal 13 is inserted into the metallic shell 1 shown in FIG. 2(a) from an insertion opening portion 1p (where a prospective crimp portion 200 which is to become the crimp portion 1d is formed) at the rear end of the metallic shell 1, thereby establishing a state in which an engagement portion 2h of the insulator 2 and an engagement portion 1c of the metallic shell 1 are engaged together via a sheet packing 63.
Then, as shown in FIG. 2(c), the ring packing 62 is disposed inside the metallic shell 1 through the insertion opening portion 1p; subsequently, the filler layer 61 of talc or the like is formed; and, furthermore, the ring packing 60 is disposed. Then, by means of a crimping die 111, the prospective crimp portion 200 is crimped to an end surface 2n of the projection 2e, which functions as a receiving portion for crimping, via the ring packing 62, the filler layer 61, and the ring packing 60, thereby forming the crimp portion 1d and fixing the metallic shell 1 to the insulator 2 through crimping as shown in FIG. 2(d). At this time, in addition to the crimp portion 1d, a groove portion 1h (FIG. 1) located between the hexagonal portion 1e and the gas seal portion 1f is also deformed under a compressive stress associated with crimping. The reason for this is that the crimp portion 1d and the groove portion 1h are thinnest portions in the metallic shell 1 and are thus readily deformable. The groove portion 1h is also called the “thin-walled portion.” After the step of FIG. 2(d), the ground electrode 4 is bent toward the center electrode 3 so as to form the spark discharge gap g, thereby completing the spark plug 100 of FIG. 1. The crimping step described with reference to FIG. 2 is of cold crimping (See Japanese Patent Application Laid-Open (kokai) No. 2007-141868); however, hot crimping (See Japanese Patent Application Laid-Open (kokai) No. 2003-257583) can also be employed.